It is important that electronic components and printed circuit boards be tested after the components have been soldered to the printed circuit boards. Several different approaches have been developed for testing the components and printed circuit boards, including functional testing, in-circuit testing, and manufacturing defect analyzers.
Functional testing uses a procedure of applying predetermined input signals and monitoring the output of a printed circuit board to determine if all of the components are present and operating properly on the circuit board. While functional testing provides a way of determining whether the P.C. board is functioning properly, it provides little or no information regarding the functioning of individual components on the board. Complex programming techniques have been used to provide limited information as to the location of non-functioning components on the board by carefully selecting input data and analyzing the output results. Such systems are complex, often costly to implement, and normally provide only vague information as to the location of malfunctioning components.
Because of the limitations of functional testing, in-circuit testing techniques have been used to individually test the components on the printed circuit board to determine if these components are working properly. This process uses a "bed of nails" tester to access each individual component and test that component individually. In this manner, non-functioning components can be identified and replaced to prevent the entire circuit board from being scrapped. This process works well for simple components where circuit inside the component is known and can be easily tested. If the component being tested is very complex, or if the circuit inside the component is unknown, in-circuit testing may not achieve satisfactory results.
Manufacturing defect analyzers are another class of testing devices that provide simpler tests and are less expensive to implement. These devices are designed to locate manufacturing faults, such as shorts on a printed circuit board, missing integrated circuits, bent component pins, etc. Although these devices do a reasonably good job of finding shorts and gross analog faults, they are marginal when testing digital sections of the board.
One very important potential problem that must be tested on every printed circuit board is whether all the pins of every component are soldered to the circuit board. Functional testing may miss a particular pin, if the functions performed by that particular pin are not thoroughly tested in functional test. Testing for this type of fault is particularly difficult when the circuit inside the component is unknown, such as the case with application specific integrated circuits (ASICs). Because of the large number of ASICs and the complexity of these devices, it is often not feasible to design an in-circuit test or a functional test to isolate this particular component.
The device of U.S. Pat. No. 4,779,041, issued Oct. 18, 1988 to Williamson, attempts to solve the above described problem. The device and test method of Williamson models a general circuit inside the device as a pair of diodes having a common resistor. This resistor is created by the wire between the device components and the external ground pin of the device. A current is passed through one of the diodes using a constant current source, to forward bias the diode. A large current pulse is then applied to the other of the diodes. The large current pulse will cause a voltage drop across the common resistor, and this voltage drop can be measured to determine whether all three pins are connected. The device of Williamson, however, suffers some limitations. Because the value of the common resistor is small, a large current pulse is needed to measure an appreciable voltage drop. Also, some current passes through adjacent devices connected to the pins, thus lowering the amount of current through the common resistor. In addition, if several pins are connected to ground in the device under test, the resistance of all these pins is placed in parallel with the common resistor, again reducing the amount of voltage which can be measured.
There is need in the art then for a device and method which will measure whether all pins of a component are soldered to a circuit board. There is further need in the art for such a device and method which does not rely on the circuit contained in the component. A still further need is for such a device and method which does not depend on a common resistor between the pins being measured and an associated ground pin. Still another need is for such a device and method which is less sensitive to adjacent devices connected to the pins.